PROJECT SUMMARY/ABSTRACT It is projected that by 2050, 3.6 million Americans will be living with a lower-limb amputation (LLA). LLA is associated with impaired balance, decreased physical function, and reduced societal participation. To date, limited research has been done to identify and evaluate modifiable factors that may contribute to impairments, activity limitations, and participation restrictions among individuals with LLAs. Given that optimal trunk function is critical for foundational activities, such as standing and walking, we believe that the trunk may be an overlooked region of impairments in these individuals. Consider that individuals following a LLA may be bed- bound for 2 weeks or more and may wait nearly 8 weeks before acquiring a prosthesis for gait training. Among healthy, younger adults, being bed-bound has been shown to negatively impact trunk muscle size and function, and long-term muscle impairments persist even after resumption of normal activities. Our previous work, among older adults, demonstrates that poor trunk muscle composition is predictive of reduced physical function, while the work of other investigators demonstrates that reduced trunk extensor endurance is associated with impaired balance and mobility, among older adults. Such relationships are not surprising as trunk muscles (i.e. multifidi, transversus abdominis, psoas, quadratus lumborum) are critical active spinal stabilizers during gait. Among individuals with a unilateral LLA, trunk compensations are common and trunk movement asymmetries are prevalent. Trunk compensations may require muscular function that is beyond the individual's capabilities given the period of immobility and muscular disuse, while trunk movement asymmetries may lead to side-to-side muscle differences. Trunk muscle impairments and asymmetries are modifiable with exercise interventions, but first, relationships between trunk muscle characteristics and function should be established. The central hypothesis of this work is that, among individuals with LLAs, trunk muscle impairments and asymmetries are present and related to poor balance, impaired physical function, and reduced societal participation. We propose a cross-sectional study of 34 individuals with a unilateral LLA [transfemoral (n=17); transtibial (n=17)] who will be compared to matched controls (n=17). The specific aims of this proposal are: (1) to determine whether impairments and asymmetries in trunk muscle function are associated with the presence of a unilateral LLA; (2) to provide the first objective magnetic resonance imaging evidence that trunk muscle morphological findings are associated with the presence of a unilateral LLA; and (3) to determine the extent to which trunk muscle characteristics are associated with balance, physical function, and societal participation among individuals with LLAs. This study will allow us to determine whether trunk muscle impairments and asymmetries are associated with the presence of a LLA. Results will inform future, longitudinal work evaluating whether trunk muscle characteristics predict balance, function, and participation, and thus, should be considered as potential modifiable factors in future interventional studies among individuals with a LLA.